1. Field of the Invention
This invention relates to a simple method of upgrading memory boards by attaching upward pin compatible memory chips with more memory capacity over the existing memory chips.
2. Description of the Prior Art
The increasing demand of denser memory chips for computers and other memory dependent devices brought forth an unprecedented improvement of such memory chips. While the state of the art DRAM chips contained 16K bits 4 years ago, today's standard are the 256Kb memory chips, with samples of 1Mb memory chips already appearing from various suppliers. This race for denser memory chips rendered memory boards obsolete within a year or two after their introduction to the market. To avoid this problem, design of the standard "4164" 64Kb DRAM chip within the 16 DIP package made use of pins no. 2 to 16, reserving pin no. 1 for use by the "4256" 256Kb DRAM chip. Under such upward pin compatibility between subsequent generations of DRAM ship design, it became possible to quadruple memory boards having 64Kb chips by replacing the 64Kb chips with 256Kb chips.
Most initial design of the memory boards with the 64Kb chips provided for easy replacement of the 64Kb chips with the new 256Kb chips by inserting the original chips in DIP sockets. Yet some boards used soldered 64Kb chips which require delicate and expansive desoldering work for replacing the 64Kb chips. For instance, such problem exists within the successful "Macintosh" computer of the Apple Computer Company. This computer has 16 units of "4164" 64Kb memory chips soldered on the 4 layer complex main logic board. The desoldering process of these 16 memory chips is very delicate and require expansive desoldering machines. With the reduction in the price of the 256Kb chips, many "Macintosh" owners had to pay three to four times the price of each chips to have their boards upgraded by professional technicians. Other solutions of the prior art include the addition of a second memory board with 256Kb chips onto the original "Macintosh" board. This solution has the disadvantage of extra cost of the additional board and there is still the need to desolder several IC chips.